Current industrial processes for the manufacture of silicone fluids, resins and rubbers typically require as starting materials either linear hydroxy endblocked polyorganosiloxanes or cyclic polyorganosiloxanes. These polysiloxanes can be produced by the hydrolysis of diorganodihalosilanes. This process results in a mixture of cyclic polyorganosiloxanes and linear hydroxy endblocked polyorganosiloxanes. Separation of this mixture to isolate a desired linear fraction or cyclic fraction results in an excess of linear materials or cyclic materials, as well as materials of undesired molecular weight. Therefore, a process which allows for converting the linear hydroxy endblocked polyorganosiloxanes and/or the linear chloro-endblocked polyorganosiloxanes to cyclic polyorganosiloxanes and that allows for adjusting the molecular weight of the polyorganosiloxane chains is desirable to allow recovery of these excess siloxanes.
Known methods for enhancing the production of cyclic polyorganosiloxanes include cracking of the linear hydroxy endblocked polyorganosiloxanes, which is capital intensive; vacuum hydrolysis, which has poor enhancement capabilities; and aqueous hydrolysis, which tends to sacrifice chloride recovery. Other methods for enhancing the production of cyclic polyorganosiloxanes require the addition of solvents and/or surfactants which makes recovery of the product more difficult and can compromise product purity. Other catalysts, such as alumina, generate undesired branched siloxane species such as CH3SiO3/2 through cleavage of organic groups from silicon.
The present process offers advantages over previously described processes as noted above. In addition, the present process can be used not only to enhance the production of cyclic polyorganosiloxanes, but also to control the cyclic polyorganosiloxane content from about zero to greater than 90 percent by weight of the product. The aqueous HCl catalyst used herein improves the rate of redistribution while minimizing organic cleavage. The linear hydroxy endblocked polyorganosiloxanes and/or the linear chloro-endblocked polyorganosiloxanes can easily be redistributed to more desirable cyclic polyorganosiloxanes, and chloride-end terminated polyorganosiloxanes typically having 2-8 siloxane units. In addition, no solvents or surfactants are required, and the aqueous HCl catalyst is readily available.